Photoelectric cell



PHOTOELECTRIC CELL Filed July 2, 1927 Patented Oct. 13, 1931 TBS ABRAHAMJ'OFFE, 0F LENINGRAD, BUSSTA, ASSIGN'OR TO INDUSTRIAL RESEARCH COM-PATENT OFFICE PANY, OF CAMBRIDGE, MASSACHUSETTS, A CORPORATION OFMASSACHUSETTS PHOTOELECTRIC CELL Application filed July 2,

substance is subjected to considerable elec trical stress, the mediumthrough which the ion travels at high velocity gives rise to anaugmentation of the number of charged particles. The accumulative actionefiectsa general movement of ions toward one of the electrodes andresults in a greatly magnified space current with abrupt reduction ofimpedance to produce amplification of the impulse originally excitingthe single ion. The original impulse may be energy derived from anyphysical phenomenon such as light, heat, eipctron bombardment or otherelectrical e cats.

The primary object of my invention is to translate energy impulses intoelectrical undulations. Another object is to provide an impedance, themagnitude of which changes in response to variations of an excitingforce. Other objects and features will be'ap-' parent as thespecification is perused in connection with the accompanying drawingwhich illustrates a preferred embodiment.

In the figure, the impedance in question comprises a sheet or layer 1 ofsuitable material interposed between electrodes 2-2. The material maycomprise a coating of linseed oil, solidified by baking or a thin layerof bakelite or other phenolic condensation product having thicknesspreferably below 10 mu or .01 mm. In case ionization is initiated bylight impinging'onthe surface of the solidified mass, the latter shouldpreferably include light sensitive material as sulphur in its naturalstate or salt previously activatedby X-rays.v This ingredient may beincorporated by way of admixture or condensation and may be uniformlydistributed throughout the body. In order that the sensitive product maybe presented to the direct 1927. Serial No. 203,023.

action of the rays, one of the electrodes, the v upper in the figure,preferably should be permeable to light, e. g. by beating gold, silver,or similar ductilemetal very thin or as a coat formed integral with thedielectric by any of the well-known sputtering or bombardment processes.The ionizing source is indicated at 5.

The operation of the device described above Wlll be apparent from aconsideration of the following discussion. In practically all substancesa number of free ions exist. These ions are also in constant motion. Theaverage distance through which an ion travels before it collides with anatom or molecule of the substance in which it occurs is called the meanfree path of the ions in that substance. If a suflicient potentialgradient exists through that substance an ion in falling through thatpotential gradient for a distance equal to its mean free path acquiressufficient kinetic energy to ionize the atom or molecule with which itcollides. If, however, the potential gradient is not suiliciently largeno ionizing collisions will occur. When a sufliciently large potentialis impressed across a substance so that ionizing collisions do occur thenumber of charged particles liberated by accumulative ionization withinthe body due to each original free ion is in the ratio of 2 to theexponent D /)t where D is the thickness of the material between thepoint at which the free ion originates and the surface of the materialto which it is attracted by the ionizing potential and A is the meanfree path of the ion in the materiah Thus it is apparent that if D isvery large as compared with the number of charged particles created isquite enormous and would result in the passage of very large currentsthrough the substance. If however D is merely several times larger than)t, the number of charged particles created bears a direct relation tothe number of original free ions but is still within a reasonable value.The number of free ions in the substances of low conductivity such asthose with which the applicant is dealing is so small as to bepracticably negligible. If, however, in accordance with the descriptionabove, D is very large with respect to a, the number of 8 is'impressed'on that su stance.

M) impressed across them. If, however, the

thickness is of the order of several times that of A, the number ofcharged particles liberated by accumulative ionization due to thepassage of an originally free ion'is of such a value that the currentpassing through the substance due to these particles is within areasonable value.

From the foregoin it will be evident that when a wave of light frequencypasses through the translucent electrode and impinges on the sensitivelayer, there is created an action within the layer setting free one ormore charged articles which are electrostatica'lly propelle toward theother electrodeunder the influence of the voltage gradient establishedby high potential source 3 (e. g. 3000 to 5000 volts). For every iondislodged in this manner, there is liberated by accumulative ionizationwithin the bod a large number of charged particles in the ratio of 2%(two to the exponent DA) where D is the thickness of dielectric, and Ais the mean free path of ions in the dielectric or material of lowspecific conductivity. Since the number of free ions within thedielectric itself is negligible, it will be seen that the number of 7charged articles originating at the light the ordinary gas-tube cell.

sensitive ayer are considerably greater than any free electrons whichmay exist within the dielectric. It is apparent that as the intensity oflight is varied, the magnitude of space current undergoes a chancommensurate with said ratio, hence amp ification of the light wavevariation is obtained in terms of current undulations :which' may betransferred through transformer 4 to a load cir-v cuit for any desiredpurpose.

Obviously this invention ofiers marked advantages over photo-electriccells of the gas tube type. The device is much more rugged and far lesssubject tobreaka The ight is not required to pass throu a glass wall andis therefore not subjecte to the absorptive action of lass upon thelight radiation, articularly o the shorter wave-lengths. The ight waves(or electrons) are incident directl u on the electrode and may evenpermeate t e electric material at least through apart of its thicknessthereby giving rise to cumulative ionization which is far more efiectiveto produce amplification than. in the case of Various ways of formingthe thin of dielectric or poorly conducting material are described in mycopending applications Serial No. 65,262, filed October 28 1925; SerialNos. 109,213 and 109,214, filed Ma 15, 1926; Serial Nos. 160,771, 160,772, filed January 12, 1927 and Serial Nos. 168,448, 168,449 and 168,450,filed February 15, 1927 which are incorporated herein by reference.

1 claim:

1. A light sensitive electrical device comprising an insulating layerhaving a thickness not greater than 10 mu, a photo-electrically activesubstance distributed through metallic sheets, at least one of saidsheets being transparent to light, and a layer of insulating materialhaving a thickness not greater than 10 mu and impregnated withphotoelectric substances.

5. A light sensitive electrical device, comprising an insulating layerhaving a thickness not greater than the order of several times the meanfree path of ions in that layer, and means associatedwith saidinsulating layer to liberate electrons by photo-electric action.

6. In an electrical device, two electrodes spaced by an insulatingmedium whose thickness is not greater than the order of several timesthat of the mean free path of ions in N that medium, means associatedwith said insulating medium to liberate electrons by photoelectricaction, and a source of potential 1m ressed across said electrodes, saidpotentia bein 'sufiiciently great to cause said electrons to iaveionizing collisions with atoms of said medium.

7 A photo-cell, comprising a pair of electrically conducting terminallayers having extended surfaces disposed adjacent to each other andseparated from each other with an insulating junction layer ofnegligible thickness, said insulating unction layer embodying substanceadapte to pass electrons from one conducting layer to the other underphotoelectric action, one of said conducting layers being transparent.

8. A photo-cell, comprisin a pair of electricall conducting terminalayers having exten ed surfaces disposed adjacent to each other andseparated from each other with an insulatin junction layer of athickness not greater 10 mu,'said insulating junc-- tion layer embodyingsubstance adapted to pass electrons from one conducting layer to theother under photo-electric action, one of said conducting ayers beingtransparent.

9. A photo-cell, comprisin a pair of electricall conducting terminalayers having extencl ed surfaces disposed adjacent to each other andseparated from each other with an. I insulating junction layer ofnegligible thickness, one of said conducting la ers bein transparent,and means associate with sai insulating junction layer to liberateelectrons by photo-electric action and pass a dischlzlarge from oneconducting layer to the ot er.

Signed by me at Cambridge, Massachusetts this 7th day of June, 1927.

ABRAHAM JOFFE.

